Helical stent having improved flexibility and expandability

ABSTRACT

A stent includes a central portion of helically wound undulations formed of struts, cylindrical end portions, and transition zones between the helical portion and the cylindrical 5 portions. According to a first aspect of the invention, the torsional flexibility of the stent is maximized by having bridges connecting adjacent winding be interrupted by the maximum possible number of undulations. In a preferred design, each winding includes nineteen undulations around the circumference, bridges are provided every five undulations. According to a second aspect of the invention, uniform, opening of the transition zone is achieved by altering 10 the width, and thereby the flexibility, of a series of struts in accordance with their lengths. Specifically, the long transition zone struts are made wider.

CROSS REFERENCE TO A RELATED PATENT APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/074,806, filed Mar. 8, 2005, which is a continuation of U.S. patentapplication Ser. No. 10/231,666, filed Aug. 30, 2002, now U.S. Pat. No.6,878,162, the disclosures of which are incorporated by reference hereinin their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates broadly to arterial prosthesis. Moreparticularly, this invention relates to vascular stents, and even moreparticularly to helical stents.

2. State of the Art

Transluminal prostheses are widely used in the medical arts forimplantation in blood vessels, biliary ducts, or other similar organs ofthe living body. These prostheses are commonly known as stents and areused to maintain, open, or dilate tubular structures.

Stents are either balloon expandable or self-expanding. Balloonexpandable stents are typically made from a solid tube of stainlesssteel. Thereafter, a series of cuts are made in the wall of the stent.The stent has a first smaller diameter configuration which permits thestent to be delivered through the human vasculature by being crimpedonto a balloon catheter. The stent also has a second, expanded diameterconfiguration, upon the application, by the balloon catheter, from theinterior of the tubular shaped member of a radially, outwardly directedforce.

Self-expanding stents act like springs and recover to their expanded orimplanted configuration after being compressed. As such, the stent isinserted into a blood vessel in a configuration after being compressed.As such, the stent is inserted into a blood vessel in a compressed stateand then released at a site to deploy into an expanded state. One typeof self-expanding stent is composed of a plurality of individuallyresilient and elastic thread elements defining a radially self-expandinghelix. This type of stent is known in the art as a “braided stent”.Placement of such stents in a body vessel can be achieved by a devicewhich comprises stent”. Placement of such stents in a body vessel can beachieved by a device which comprises an outer catheter for holding thestent at its distal end, and an inner piston which pushes the stentforward once it is in position. However, braided stents have manydisadvantages. They typically do not have the necessary radial strengthto effectively hold open a diseased vessel. In addition, the pluralityof wires or fibers used to make such stents could become dangerous ifseparated from the body of the stent, where it could pierce through thevessel.

Therefore, recently, self-expanding stents cut from a tube ofsuperelastic metal alloy have been manufactured. These stents are crushrecoverable and have relatively high radial strength. U.S. Pat. No.5,913,897 to Corso, U.S. Pat. No. 6,042,597 to Kveen, and WPO PatentApplication WO 01189421-A2 (with inventors Cottone and Becker, andreferred to herein as “Cottone”) each teach superelastic cut-tubularstents having a helically wound configuration of repeating undulations.Bridge structures connect adjacent circumferential windings by extendingbetween loop portions of undulations on adjacent windings. However, thebridge structures and arrangements do not maximize the torsionalflexibility of the stents. In particular, Cottone describes a stenthaving a helical pattern of bridges (connections) connecting windings ofthe helix which is reverse in handedness from the undulations of thewindings which form the central portion of the stent. The designdescribed provides the stent with asymmetric characteristics that causethe stent to resist torsional deformations differently in one directionversus the other. In addition, each “helix of connections” forms astring of connections in which the connections are interrupted by onlyone and one-half undulations. As such, that string is resistant tostretching and compression. Accordingly, when a stent so designed istwisted torsionally, that string of connections causes constriction ofthe stent when twisted in the “tightening” direction (i.e., in thedirection of the windings) and expansion of the stent when twisted inthe opposite “loosening” direction. This differential torsional reactionresults in the undulations of the stent being forced out of thecylindrical plane of the surface of the stent, such that the stentappears to buckle when twisted in the “loosening” direction.

In fact, even if the stent were constructed opposite to Cottone'spreferred embodiment (that is, with a helix of bridges having the samehandedness as the helix of undulations), the same effect results. Stentsbuilt with constructions containing a string of bridges separated byonly a small number of undulations behave poorly when twisted. That is,they react differently if the stent is twisted one way versus the other,and the surface of the stent tends to buckle when twisted only slightlyin the “loosening” direction.

Moreover, due to the helical windings of the stents, the stentsdescribed by Corso and Kveen terminate unevenly at the end of thehelical windings. As such, the terminus of the final winding fails toprovide a uniform radial expansion force 360 therearound. Cottoneaddresses this problem by providing a stent constructed with a helicallywound portion of undulations in the central portion of the stent, acylindrical portion of undulations at each end of the stent, and atransition zone of undulations joining each cylindrical portion to thecentral helically wound portion. The undulations of the transition zoneinclude struts which progressively change in length.

Because the transition zone must mate directly to the cylindricalportion on one side and to a helically wound portion on the other side,the transition zone must create a free end from which the helicalportion extends, must contain a bifurcation, and must depart from auniform strut length for the struts around the circumference of thetransition zone so that the transition from the helically wound portionto the cylindrical portion can occur.

However, if there are longer struts in a portion of the transition zone,that portion tends to expand more than the portion with shorter strutsbecause the bending moments created by longer struts are greater thanthose created by shorter struts. Also, for the same opening anglebetween two such struts when the stent is in an expanded state, theopening distance between such struts is greater if the struts arelonger. These two factors combine their effects in the portion of thetransition zone with longer struts so that the apparent openingdistances are much larger than in the portion where the struts areshorter. As such, the simple transition zone described by Cottone is notamenable to uniform expansion and compression, which is a requirement ofan efficient self-expanding stent.

Moreover, except in the case of the Cottone helical stent which isprovided with a transition zone, and except where there are differentstrut lengths in the undulations at the ends of a stent, stentsgenerally contain struts of one length throughout their design.Accordingly, in order to achieve uniform opening of the stent, all thestruts have substantially the same width as well as length.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a cut-tubeself-expanding helical stent which has substantially equal torsionalflexibility and resistance to torsional buckling when twisted in bothdirections.

It is another object of the invention to provide a cut-tubeself-expanding helical stent having a transition zone and a cylindricalsegment at each end thereof, and to improve the expandability of thetransition zone.

It is a further object of the invention to provide a cut-tubeself-expanding helical stent having a transition zone in which openingscreated between the struts of an expanded stent can be made more uniformover the entire transition zone.

In accord with the invention, which will be described in detail below, acut-tube self-expanding stent having a central helically wound portioncomprising repeating undulations formed of struts is provided at each ofits ends with a cylindrical portion, and a transition zone between thehelical portion and each cylindrical portion.

According to a first aspect of the invention, several criteria are setforth which together provide for optimal torsional flexibility andexpandability in a self-expanding helically wound stent. According to afirst criterion, the torsional flexibility of the stent is maximized byhaving all the “strings” of bridges which connect adjacent helicalwinding be interrupted by the maximum possible number of undulations.This results in these bridge strings being as stretchy and compressibleas possible. According to a second criterion, the undulations in thecentral portion are interdigitated. According to a third criterion,preferred numbers of undulations, bridges, and undulations betweenbridges are provided. According to a fourth criterion, the bridgespreferably extend in a “short” direction, longitudinally crosswiseacross the helically space separating the helical windings ofundulations. Most preferably, the bridges join loops of undulationswhich are out of phase by one and one-half undulations.

According to a second aspect of the invention, uniform opening of thetransition zone is achieved by altering the flexibility of a series ofstruts in accordance with their lengths. Specifically, the longtransition zone struts are made wider (in the cylindrical plane) tocompensate for the greater bending moments imposed by the longer struts.This keeps the opening distance (the distance between the open ends ofadjacent struts in an expanded stent) approximately constant throughoutthe transition zone. More particularly, in a typical transition zone,the shortest strut must be approximately half the length of the longeststrut. In order to maintain similar opening distances, the long strutsshould be wider by approximately the cube root of 2 squared, i.e.approximately 1.59. The ratio may be adjusted to a value near this ratioin order to achieve a uniform opening, giving consideration to the factthat in a transition zone two adjacent struts of unequal length bothcontribute to the bending moment on the flexing connection that joinsthem. The ratio may also be adjusted to make the opening angle of theshortest strut pairs not exceed a certain value in order to limit themaximum strains experienced in that portion of the transition zone.

Additional objects and advantages of the invention will become apparentto those skilled in the art upon reference to the detailed descriptiontaken in conjunction with the provided figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a broken flattened view of a helical stent according to theinvention in an unexpanded state, wherein the stent has been cutparallel to its longitudinal axis and laid flat;

FIG. 2 is an enlarged broken flattened view of a central portion of thehelical stent of FIG. 1;

FIG. 3 is an enlarged broken flattened view of a transition zone portionof the helical stent of FIG. 1; and

FIG. 4 is a schematic view of a plurality of struts of the transitionzone of FIG. 3 shown in an open configuration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to FIG. 1, a helical stent 10 according to the invention isshown. The stent has a collapsed configuration with a first smallerdiameter for insertion into a body vessel, and self-expands to anexpanded or deployed configuration with a second larger diameter fordeployment within the vessel. The stent is preferably a laser-cuttubular construction of a superelastic metal alloy such asnickel-titanium.

The stent 10 includes a central portion 12, a cylindrical portion 14 ateach end of the stent 10, and a transition zone 16 between the centralportion 12 and each cylindrical end portion 14. The central portion 12is comprised of a plurality of helical circumferential windings (singleturns of a helix) 18 of substantially like undulations (in length andwidth) 20, with each undulation 20 being defined by two adjacent struts,e.g., struts 22, 24, and a loop 26 connecting the struts (FIG. 2). Thecylindrical end portion 14 is comprised of preferably a singlecylindrical winding 28 of like undulations 30, with each such undulation30 being defined by two adjacent struts, e.g., struts 32, 34, and a loop36 connecting the struts. Optionally, one or more structures 37 adaptedto receive or otherwise be coupled to radiopaque markers (not shown) canbe provided at the ends of one or more of the undulations 30. Thetransition zone 16 is comprised of preferably a single winding 38 ofundulations 40 that preferably progressively increase in size, with eachsuch undulation 40 being defined by two adjacent struts, e.g., struts42, 44, and a loop 46 connecting the struts.

In each of sections 12, 14 and 16, the undulations 20, 30, 40 extend ina generally longitudinal direction. That is, when the stent is in acollapsed configuration, as shown in FIG. 1, struts of the helicalportion (e.g., 22 and 24), cylindrical portion (e.g., 32 and 34) andtransition zone (e.g., 42 and 44) all extend substantially parallel tothe longitudinal axis AL of the stent. In the expanded configuration,adjacent struts are moved apart and angled relative to each other.

Referring to FIG. 2, particularly with respect to the central portion12, as the windings 18 a, 18 b, 18 c are longitudinally displaced alongthe length of the stent, bridges, e.g. 50, 52, 54 and 56, are providedto connect together the loops 26 of undulations 20 on adjacent windings,e.g. 18 a and 18 b, and 18 b and 18 c, to prevent stent unwinding. Thebridges 50, 52, 54, 56 can be seen to be arranged in right-handed andleft-handed helical “strings” (right-handed string 60 and left-handedstring 62) which extend about the stent.

There are several preferred design considerations (criteria) which,according to the invention, are preferably used together to identify adesired number and placement of undulations in any winding and a numberand placement of bridges 50, 52, 54, 56 which connect together loops 26of undulations on adjacent windings (and thereby connect together thewindings 18 a, 18 b, 18 c). If the central portion 12 is designed inaccord with the following criteria, the central portion 12 will have adesired torsional flexibility and expandability; i.e., be not too stiff,but also be sufficiently flexible so that the central portion 12 willnot be subject to kinking.

In accord with a first criterion, the pattern of bridges is as symmetricas possible. That is, the right-handed and left-handed strings 60, 62 ofbridges should be as similar as possible. Further, the torsionalflexibility of the stent is maximized by having each string 60, 62 ofbridges be interrupted by the maximum possible number of undulations 20.This results in the bridge strings being as stretchy and compressible aspossible. In any given stent design, there is a certain number ofundulations which form a complete circumferential winding (single turnsof the helical portion). The number of undulations 20 which separate thebridges lying along any one string depends, therefore, on the number ofbridges within a complete circumferential winding. For example, if thereare eighteen undulations around a circumferential winding and threebridges, and if the bridges on adjacent windings are staggered, inaccord with the invention there should be three undulations separatingbridges along each helical strings of bridges.

In accord with a second criterion, it is preferred that the loops 26 ofthe undulations 20 of the central portion 12 be interdigitated betweenthe loops of the undulations on an adjacent winding. For example, ifthere are eighteen undulations around the circumference, each undulationwould be rotationally displaced from the undulations on the adjacentwinding by one-half an undulation (i.e., one thirty-sixth of a circle orten degrees), so that the “peak” of one loop is directed into the“valley” between two loops on an adjacent winding.

In accord with a third criterion, it is necessary to observe how thenumber (m) of undulations between bridges and the number (n or n+½) ofundulations around the circumference interact to create helical stringsof bridges. That is, with an increase in n for a stent of a givendiameter, the stent is weakened and subject to kinking. This is because,for a stent of a given diameter, by providing more struts, narrower andweaker struts must be used. As n is decreased, the struts are increasedin width and thus stiffness. However, while this may strengthen thestent, the stent is nevertheless limited in flexibility and may beundesirably stiff. In accord with the invention, for the optimumcombination of strength and flexibility, it is preferred that n (i.e.the number of undulations) be sixteen to twenty, and more preferablyeighteen to nineteen, where n may optionally be a non-integer. Inaddition, the number of bridges, m, for the preferred number of strutsis most preferably three to five bridges per circumferential winding.

In accord with a fourth criterion, consideration must be made as to thelocations between which the bridges connect and the direction in whichthe bridges extend. In accord with the preferred interdigitatedcriterion, the bridges cannot extend parallel to the longitudinal axisAL of the stent. Rather, they preferably extend across loops located oneand one-half pitches away; i.e., each bridge connects over two strutsrelative to directly across from the strut from which the bridgeextends. In addition, the bridges extend longitudinally crosswise acrossthe helical space separating the adjacent loops (i.e. in a “short”direction), as opposed circumferentially along the helical spaceseparating the adjacent loops (i.e., in a “long” direction).

In view of the above, a preferred exemplar central portion 12 of thestent 10 illustrating the application of the above criteria is nowdescribed. Referring to FIG. 2, the central portion 12 of the stent 10includes repeating undulations 20 (each comprising two struts 22, 24 anda loop 26) that are helically wound in circumferential windings 18 a, 18b, 18 c, etc. There are preferably nineteen undulations 20 in eachcircumferential winding 18 a, 18 b, 18 c and the undulations areinterdigitated. With reference to windings 18 b and 18 c, a bridge 50,52, 54 is located every five undulations therebetween, and each bridgejoins loops of undulations on the adjacent windings 18 a, 18 b which areone and one-half pitches away (or two struts over from directly across)in the “short” direction. That is, all bridges in the central portion 12of the stent preferably extend in the same direction, longitudinallycrosswise across the helical space. This preferred exemplar embodimentprovides a very symmetrical distribution of bridges. In particular,traveling from anyone bridge, e.g. bridge 54, to the next bridge, e.g.bridge 56, along the right-hand string 60 of bridges, traverses exactlytwo and one half undulations (or five struts 70, 72, 74, 76 and 78).Moreover, traveling from anyone bridge, e.g. bridge 52, to the nextbridge, e.g. bridge 56, along the left-handed string 62 of bridges, alsotraverses exactly two and one half undulations (or five struts 80, 82,84, 86 and 88). This design gives very even torsional flexibility andexpandability, and the stent may be twisted considerably in eitherdirection without buckling.

Referring now to FIG. 3, the transition zone 16 of the stent 10 isshown. The transition zone, as stated above, includes struts thatprogressively increase in length. The long transition zone struts 90, 92are relatively wider (in the cylindrical plane) than the shortertransition zone struts 94, 96 to compensate for the greater bendingmoments imposed by the longer struts. Moreover, even the shortesttransition zone strut 98 is preferably longer and wider than the struts22, 24 of the central portion 12.

More particularly, referring to FIGS. 3 and 4, for substantially uniformexpansion of the stent, it is desirable for the opening distance D(i.e., the distance between the ends of two adjacent struts, e.g. struts32 and 34, when the stent is in an open configuration) to beapproximately even throughout the transition zone 16. Accordingly, theopening angle a between pairs of longer struts in the transition zone,e.g. struts 32 and 34, must be smaller than the opening angle a betweenshorter struts, e.g. struts 94 and 96. In addition, the bendingstiffness of the longer struts must be even greater than in proportionto their increased bending moment. The bending stiffness S of arectangular-section beam is in proportion to the third power of thewidth (in the bending direction) W. As such, by way of example, in orderto double the bending stiffness of a strut, the width W of the strutmust be increased by the cube root of two.

The bending moment M of a strut is in linear proportion to the length Lof the strut. The opening angle a is proportional to the bending momentM divided by the stiffness S. The opening distance D is proportional tothe product of the opening angle a multiplied by strut length L.Therefore, the opening distance D is proportional to a*L, which is equalto (MIS)*L. Since M is linearly proportional to L, the opening distanceD is proportional to the square of L divided by stiffness S. In order tokeep the opening distance D between adjacent struts (i.e., pairs ofstruts) equal throughout the transition zone 16, the stiffness of thebending segments of the struts must be in proportion to the square oftheir lengths. Hence, the cube of the width must be proportional to thesquare of the length:

W³ is proportional to L²,

In a preferred transition zone, the shortest strut 98 should beapproximately half the length of the longest strut 42. Therefore, inorder to maintain similar opening distances, the longer struts are mostpreferably wider by the cube root of 2 squared, or 1.59, relative to theshorter struts. The ratio may be adjusted to a value near this ratio(e.g., +25%, or 1.19 to 1.99) in order to achieve a uniform opening,giving consideration to the fact that in a transition zone two adjacentstruts of unequal length both contribute to the bending moment on theflexing connection that joins them. It may also be desirable to make theopening angle a between the shortest strut pairs not exceed a certainvalue in order to limit the maximum strains experienced in that portionof the transition zone.

As such, uniform opening is achieved in the transition zone by alteringthe flexibility of a series of struts in accordance with their lengths.

There have been described and illustrated two aspects of a preferredstent relating to the helical central portion and the transition zone.While particular embodiments of the invention have been described, it isnot intended that the invention be limited thereto, as it is intendedthat the invention be as broad in scope as the art will allow and thatthe specification be read likewise. Thus, the two preferred aspects(relating to the central helical portion and the transition zone) can beused together or separately. Moreover, each of the design considerationsrelating to the helical central portion can be used alone or togetherwith one or more of the other considerations. It will therefore beappreciated by those skilled in the art that yet other modificationscould be made to the provided invention without deviating from itsspirit and scope as claimed.

1. A stent for insertion into a vessel of a patient, comprising: atubular member having a first smaller diameter for insertion into thevessel, and a second larger diameter for deployment within the vessel,the tubular member including a central portion comprised of a pluralityof helical circumferential windings separated by a helical space, eachof the windings including a plurality of undulations, with eachundulation being defined by two adjacent struts and a loop connectingthe struts, wherein adjacent windings are connected with a plurality ofbridges, the bridges extending across the helical space between adjacentwindings and connected to the loops of a plurality of the undulations,and further wherein a same number of struts is traversed from any onebridge to a next bridge on each of the windings when traveling in eithera right-handed or a left-handed direction along each of the windings,and wherein the tubular member includes cylindrical end portions, and atransition zone between the central portion and each of the endportions, the transition zone including a plurality of transitionundulations having struts that progressively increase in length, whereina relatively longer strut of the transition undulations has a relativelywider width than a relatively shorter strut of the transitionundulations, and wherein the transition zone defines struts that arelonger than a shortest strut of the transition zone, each of the longerstruts having opposing ends connected to loops adjoining adjacentstruts, a terminal end of a strut of the central portion adjoining atleast one longer strut between the opposing ends of the at least onelonger strut.
 2. The stent of claim 1, wherein the loops of theundulations on adjacent windings are interdigitated.
 3. The stent ofclaim 2, wherein the bridges extend between loops located one andone-half pitches away.
 4. The stent of claim 1, wherein the centralportion of the stent includes sixteen to twenty undulations.
 5. Thestent of claim 4, wherein the central portion of the stent includeseighteen to nineteen undulations.
 6. The stent of claim 1, wherein eachwinding includes three to five bridges extending therefrom.
 7. The stentof claim 1, wherein each of the bridges in the central portion extendsin a same direction in a cylindrical plane of the stent.
 8. The stent ofclaim 7, wherein each of the bridges extends longitudinally crosswiseacross the helical space.
 9. The stent of claim 1, wherein the tubularmember is self-expanding from the first diameter to the second diameter.10. The stent of claim 1, wherein the tubular member is a laser cuttube.
 11. The stent of claim 1, wherein the tubular member is made froma superelastic material.
 12. The stent of claim 1, the at least onelonger strut being a longest strut of the transition zone.
 13. A stentfor insertion into a vessel of a patient, comprising: a tubular memberhaving a first smaller diameter for insertion into the vessel, and asecond larger diameter for deployment within the vessel, the tubularmember including a central portion comprised of a plurality of helicalcircumferential windings separated by a helical space, each of thewindings including a plurality of undulations, with each undulationbeing defined by two adjacent struts and a loop connecting the struts,wherein adjacent windings are connected with a plurality of bridges, thebridges extending across the helical space between the adjacent windingand connected to the loops of a plurality of the undulations, wherein,for at least one of the windings, for any given number n of undulationson the winding and any given number b of bridges connected to thewinding, where n is not equal to b, a maximum number of undulations isprovided between the bridges, the tubular member also having atransition zone between the central portion and a cylindrical endportion, the transition zone including a plurality of transitionundulations having struts that progressively increase in length, whereina relatively longer strut of the transition undulations has a relativelywider width than a relatively shorter strut of the transitionundulations, wherein the transition zone defines struts that are longerthan a shortest strut of the transition zone, each of the longer strutshaving opposing ends connected to loops adjoining adjacent struts, aterminal end of a strut of the central portion adjoining at least onelonger strut between the opposing ends of the at least one longer strut.14. The stent of claim 13, wherein the undulations on adjacent windingsare interdigitated.
 15. The stent of claim 13, the at least one longerstrut being a longest strut of the transition zone.
 16. A stent forinsertion into a vessel of a patient, comprising: a tubular memberhaving a first smaller diameter for insertion into the vessel, and asecond larger diameter for deployment within the vessel, and defining alongitudinal axis, the tubular member including a central portioncomprised of a plurality of helical circumferential windings separatedby a helical space, each of the windings including a plurality ofundulations, with each the undulation being defined by two adjacentstruts and a loop connecting the struts; at least one cylindrical endportion at an end of the central portion; and a transition zone betweenthe central portion and each of the cylindrical end portions, thetransition zone including a plurality of transition undulations havingstruts that progressively increase in length, wherein a relativelylonger strut of the transition undulations has a relatively wider widththan a relatively shorter strut of the transition undulations, thetransition zone defining struts that are longer than a shortest strut ofthe transition zone, each of the longer struts having opposing endsconnected to loops adjoining adjacent struts, a terminal end of a strutof the central portion adjoining at least one longer strut between theopposing ends of the at least one longer strut.
 17. The stent of claim16, wherein the tubular member has a longitudinal axis and adjacentstruts of the transition zone are defined as a pair of struts, thetransition zone having a plurality of the pairs, and when the stent hasthe first smaller diameter, the struts of the central portion and thetransition zone all extend substantially parallel to the longitudinalaxis, and when the stent has the second larger diameter, an angle isdefined between the struts of each pair of struts, and the angle issubstantially the same for each pair of struts.
 18. The stent of claim16, wherein the relatively longer strut of the transition undulations is1.19 to 1.99 times wider than the relatively shorter strut of thetransition undulations.
 19. The stent of claim 16, wherein therelatively longer strut of the transition undulations is approximately1.59 times wider than the relatively shorter strut of the transitionundulations.
 20. The stent of claim 16, wherein the at least onecylindrical end portion is two cylindrical end portions.
 21. The stentof claim 16, wherein all of the struts of the central portion have acommon length and a common width.
 22. The stent of claim 21, wherein thecommon length of the struts of the central portion is shorter than theshortest strut of the transition zone, and the common width of thestruts of the central portion is narrower than a narrowest strut of thetransition zone.
 23. The stent of claim 16, the at least one longerstrut being a longest strut of the transition zone.